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Coaxial Cable length for impedance match

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Newbie level 5
May 5, 2007
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I have a 70 Mhz VHF generator having maximum power rating of 1.5KW. The generator is connected to the matching box with a coaxial cable. The matching box is connected directly to the plasma chamber.

The question that I had was what should be the length of the cable from the generator to the matching box so that the line acts as a 50 ohm line. Sould it be λ/4 (quarter wavelength) , λ/2 (half) or λ.

multiples of a quarterwave but not a full wave long

Added after 51 seconds:

multiples of a quarterwave but not a full wave long

so 1/4wave / velocity factor of the cable this is about 0.96 for 50ohm rg59u

cable quality is shit

i use 2X swr meter and a dummy load one at tx end other at dummy load
to trim for 1:1 both ends cut about 1/2 " off at a time
For 70MHz, RG393 (VF 0.694):

1/4 wavelength = 246/70 MHz = 3.514 feet long (electrical) x 0.694 VF = 2.43

1/4 wavelength is 2.43' or 29.26" so an odd
multiple is 3 x 29.26" or 87.8" length. Does this sound good?

You have to give more details:

- what is the output impedance of the generator? 50 ohm?

- what is the input impedance of the matching box? 50 ohm?
The output impedance of the generator is 50 ohm. Also I am using a 7/16 connector at the input of the matching box which is also a 50 ohm.

In that case you can use any length of the coaxial cable between the generator and the matching box. But be careful with the loss. As an example it is about 0.5 dB/10 m length at 70 MHz for RG-213/U coaxial cable.

in my opinion the answer of VSMVDD is not correct.
It's well known that a λ/2 or λ long coax line take no changes on the impedance, even regardeless its Zo.

As opposite, λ/4 acts the strongest Z transformation.

for N λ/2 (N=1,2,3..) Zin = Zload regardeless for Zo
for N λ/4 (N=1,3,5..) Zin = Zo²/Zload

Since the your power is very high, you should connect very well the brian before connect the cables :)

Man, what are you guys talking about? VSWR has the answer. You can put any length of characteristic impedance 50 ohm cable between your 50 ohm output generator and the matching box (assuming the matching box is actually working!).

If both impedences are 50 Ohm any lenght of cable is OK,you have to take care only about the maximum power permitted on the cable. ( I suggest RG214 ).
If the impedance is different any lambda/2 or multiples will transfer the impedance of the generator to the load, any impedance of the cable used.


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